On identifying don´t care inputs of test patterns for combinational circuits

Given a test set for stuck at faults, some of primary input values may be changed to opposite logic values without losing fault coverage. We can regard such input values as don´t care (X). In this paper, we propose a method for identifying X inputs of test vectors in a given test set. While there are many combinations of X inputs in the test set generally, the proposed method finds one including X inputs as many as possible, by using fault simulation and procedures similar to implication and justification of ATPG algorithms. Experimental results for ISCAS benchmark circuits show that approximately 66% of inputs of un-compacted test sets could be X in average. Even for compacted test sets, the method found that approximately 47% of inputs are X. Finally, we discuss how logic values are reassigned to the identified X inputs where several applications exist to make test vectors more desirable.